In conventional radar devices, a radar transmitter directs a beam in a direction, and transmits a plurality of transmission pulses at predetermined pulse repetition interval (PRI). A radar receiver performs coherent integration on a plurality of reflected pulses, which have been reflected, scattered, or diffracted by an object. The coherent integration is an operation of coherently integrating reflected pulses in the same range. Generally, a discrete Fourier transform (DFT) is used in coherent integration. The coherent integration process using the DFT may be generically called a Doppler filtering process. Generally, a period in which a radar transmitter transmits a plurality of transmission pulses in one beam position and a radar receiver performs coherent integration on reflected pulses based on the transmission pulses from the radar transmitter is called a coherent processing interval (CPI). The radar receiver measures the strength of a received signal coherently integrated for each CPI, and detects the received signal as a target signal when the measured value exceeds a threshold.
When the radar cross section (RCS) of the target is small, the power of the reflected pulses received at the radar receiver is small. Therefore, a target of small RCS has been very difficult to be detected even if reflected pulses are coherently integrated for each CPI.